The protest, made through diplomatic channels, came after Tokyo confirmed that the drilling ship in January moved to a location a few kilometers northeast of where it was in mid-November, in apparent search of resources.

"It's extremely regrettable that China has continued its unilateral development activity," Chief Cabinet Secretary Yoshihide Suga said at a press briefing.

Beijing has developed 16 structures on the Chinese side of a Tokyo-proposed median line separating the two countries' economic zones in the East China Sea.

Japan and China agreed on joint gas development in the area in 2008 but negotiations were suspended in 2010.

Japan fears China's unilateral development in the area may lead to the siphoning off of resources from beneath the Japanese side of the line.

The revelation came despite a recent improvement in Sino-Japanese relations.

In the East China Sea, Tokyo and Beijing have been at loggerheads over the ownership of the Senkaku Islands, administered by Japan but claimed by China.

The ship in question was found engaged in apparent drilling in September. It then moved in mid-November before its most recent change of location.